A pneumatic circuit has a pressure control valve for reducing high-pressure air from a compressor to supply it to an actuator and a proportional control valve for controlling air quantity supplied to the actuator. The pneumatic circuit also has an opening/closing control valve for opening and closing an air passage and a directional control valve for varying an air supply direction. The control valves, such as the opening/closing control valve, used for the pneumatic circuit are of the type to bring a valve element made of rubber or metal into contact with an annular valve seat at a right angle thereto. There are such control valves of the poppet and diaphragm types.
As described in Japanese Patent Application Laid-Open publication No. 2002-286157, in the opening/closing control valve of the diaphragm type, a control valve element, that is, a valve housing, is formed with an inflow passage and an outflow passage, the inflow passage and the outflow passage communicate with each other through a communication hole formed in the valve housing, and the opening end of the communication hole has an annular valve seat. A diaphragm moving to the position in which it is brought into contacts with the valve seat to close the communication hole and to the position in which it moves away from the valve seat to open the communication hole is fitted as a valve member in the control valve element.
In the control valves of the poppet and diaphragm types, used for the pneumatic circuit, in order not to cause air leakage between the valve seat and the valve member in the state that the valve member is in contact with the valve seat, the contact surface of the valve seat needs to be machined at high accuracy. In addition, in the control valve used for the pneumatic circuit, in order not to cause air leakage between the valve seat and the valve member, the valve seat is annularly projected to the valve member and its surface is formed in arcuate shape.
To machine an end face having an arcuate cross section on the annular valve seat of the valve housing, conventionally, after the communication hole is cut into the valve housing a drill, the end face of the valve seat is cut using a cutting tool. As described in Japanese Patent Application Laid-Open publication No. 64-64702, there is a cutting tool for superfine cutting.